Uptake of nitrogen by rice studied with a 15N point-placement technique
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The main objective of the work described in this paper was to investigate the usefulness of a point-placement technique for investigating nitrogen-use by irrigated rice in a tropical lowland environment. The technique was used to deliver a small quantity of urea 15N to a specific location adjacent to the roots of a rice plant in a few minutes. The amount of 15N delivered was equivalent to 4–8 h of the daily nitrogen uptake requirement of a rice plant. The 15N was placed in a gelatin capsule attached to a wooden handle and the capsule was inserted adjacent to the roots at the soil water interface or at 5 cm soil depth beneath the rice plant. The gelatin dissolves in a few minutes. The plants were harvested at varying intervals through to crop maturity and their 15N content measured. The uptake of 15N followed an exponential pattern and it took approximately two weeks to reach a maximum of about 65% of the initial quantity supplied. The time to reach maximum uptake was unexpectedly slow and a simple theory was developed to better understand the processes governing the uptake and loss of the 15N. There were no differences between the applications made at the soil surface and those made 5 cm below the surface. The technique worked very well for tracking the fate of nitrogen acquired by the plant at different growth stages. For example, 14 days after label application at 10 days after transplanting (DAT), 66% of the labeled N acquired by the plant was found in the leaves, 26% in the stems and sheaths, and 8% in the roots. About 37% of the label supplied on 10 DAT was located in the grain at maturity suggesting the technique can be used to follow the nitrogen dynamics of irrigated rice crops.
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